The invention relates generally to phosphor materials, and particularly to red emitting phosphors. More particularly, the invention relates to moisture-resistant phosphors, and a lighting apparatus employing these phosphors.
Light emitting diodes (LEDs) are semiconductor light emitters often used as a replacement for other light sources, such as incandescent lamps. They are particularly useful as display lights, warning lights and indicator lights, or in other applications where colored light is desired. The color of light produced by an LED is dependent on the type of semiconducting material used in its manufacture.
Colored semiconductor light emitting devices, including light emitting diodes and lasers (both are generally referred to herein as LEDs), have been produced from Group III-V alloys such as gallium nitride (GaN). With reference to the GaN-based LEDs, light is generally emitted in the UV-to-green range of the electromagnetic spectrum. Until quite recently, LEDs have not been suitable for lighting uses where a bright white light is needed, due to the inherent color of the light produced by the LED.
Recently, techniques have been developed for converting the light emitted from LEDs to useful light for illumination purposes. In one technique, the LED is coated or covered with a phosphor layer. A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum, and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of high chemical purity, and of controlled composition, to which small quantities of other elements (called “activators”) have been added to convert them into efficient fluorescent materials. With the appropriate combination of activators and inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range.
By interposing a phosphor excited by the radiation generated by the LED, light of a different wavelength, e.g., in the visible range of the spectrum, may be generated. Colored LEDs are often used in toys, indicator lights and other devices. Continuous performance improvements have enabled new applications for LEDs of saturated colors in traffic lights, exit signs, store signs, and the like.
In addition to colored LEDs, a combination of LED generated light and phosphor generated light may be used to produce white light. The most popular white LEDs consist of blue emitting GaInN chips. The blue emitting chips are coated with a phosphor that converts some of the blue radiation to a complementary color, e.g. a yellowish emission. Together, the blue and yellowish radiation produces a white light. There are also white emitting LEDs that are designed to convert the UV radiation to visible light. These LEDs utilize a near UV emitting chip and a phosphor blend including red, green and blue emitting phosphors, and produces white light.
In many white light applications, phosphors with line emission spectra (e.g., Y2O3: Eu3+) are preferred as the red component, because they maximize the luminous efficacy of radiation (LER) at acceptable color-rendering index (CRI) values (e.g. 80-86), over the correlated color temperature (CCT) range of interest (e.g. 3000-6500K). These red fluorescent lamp phosphors, doped with Eu3+, cannot be used successfully in UV LED lamps because they have virtually no absorption of near-UV (370-420 nm) light, leading to unacceptable light loss, due to scattering by the phosphor. Currently, a class of phosphors based on manganese (Mn4+) doped complex fluorides can be used in LED lamps, due to having their main emission peak at a wavelength ranging from 300 nm to 520 nm. These fluoride phosphors usually have high quantum efficiency, and their narrow red line emission leads to potential use in warm white light. Warm white LEDs (CCT<4500K) have high CRI (>80), and also have a high lumen equivalent.
However, these fluoride phosphors are sensitive to moisture, and degrade under high temperature (more than about 60 degrees Celsius) and high humidity conditions. The phosphor often turns brown, probably due to hydrolysis of MnF6−2 ion to hydrated manganese dioxide, which leads to significant deterioration of the brightness of these phosphors.
It would therefore be desirable to produce fluoride phosphors that are resistant to moisture-induced degradation, in order to prolong the light emission of LEDs. Moreover, the new fluoride phosphor can provide the advantage of simple packaging methods, e.g., those that may require less of a hermetic seal. It would also be very desirable to develop an improved LED that incorporates such moisture-protected phosphors.